#### The physics of causality

•  Causality has been the subject of many genre works and was perhaps most famously discussed in the Wachowskis' Matrix trilogy.

Essentially, causality is the idea that events in the present are caused by events in the past and, in turn, act as causes for what happens in the future. So for example, if event A is a cause of an effect B, then B cannot be a cause of A.

However, theoretical physicists at the University of Vienna and the Université Libre de Bruxelles recently came up with a quantum mechanic theory that says it is possible to conceive situations in which a single event can be both a cause *and* an effect of another one.   Although it is still unknown if such situations can be actually found in nature, the sheer possibility that they could exist may have far-reaching implications for the foundations of quantum mechanics, quantum gravity and quantum computing.

According to Caslav Brukner from the University of Vienna, events are ordered in time in everyday life and in classical physics. Meaning, a cause can only influence an effect in its future not in its past.

As a simple example, imagine a person, Alice, walking into a room and finding there a piece of paper. After reading what is written on the paper Alice erases the message and leaves her own message on the piece of paper.   Another person, Bob, walks into the same room at some other time and does the same: he reads, erases and re-writes some message on the paper. If Bob enters the room after Alice, he will be able to read what she wrote; however Alice will not have a chance to know Bob's message.

In this case, Alice's writing is the "cause" and what Bob reads the "effect." Each time the two repeat the procedure, only one will be able to read what the other wrote. Even if they don't have watches and don't know who enters the room first, they can deduce it by what they write and read on the paper. For example, Alice might write "Alice was here today," such that if Bob reads the message, he will know that he came to the room after her.

So as long as only the laws of classical physics are allowed, the order of events is fixed: either Bob or Alice is first to enter the room and leave a message for the other person. When quantum mechanics enters into play, however, the picture may change drastically.

According to quantum mechanics, objects can lose their well-defined classical properties, such as e.g. a particle that can be at two different locations at the same time. In quantum physics this is called a "superposition."

But now Brukner and his team say that even the causal order of events could be in such a superposition.

"If - in our example - Alice and Bob have a quantum system instead of an ordinary piece of paper to write their messages on, they can end up in a situation where each of them can read a part of the message written by the other," he explained.  "Effectively, one has a superposition of two situations: 'Alice enters the room first and leaves a message before Bob' and 'Bob enters the room first and leaves a message before Alice.'"

Such a superposition, says Ognyan Oreshkov from the Université Libre de Bruxelles, has not been considered in the standard formulation of quantum mechanics since the theory always assumes a definite causal order between events.

"[However], if we believe that quantum mechanics governs all phenomena, it is natural to expect that the order of events could also be indefinite, similarly to the location of a particle or its velocity," noted Fabio Costa from the University of Vienna. "[Of course], the real challenge is finding out where in nature we should look for superpositions of causal orders."